1. Field of the Invention
The present invention relates generally to a data processing system including a redundant array of disk drives and more particularly to a method and apparatus for processing data in a redundant array of independent disk drives (RAID) using asymmetric mirroring.
2. Description of the Prior Art
Various types of storage units, such as direct access storage devices (DASDs) are used to store data for known data processing systems. One often used type of DASD is a magnetic disk unit including a number of disks having surfaces with magnetic active material onto which data is written and from which data is read by magnetic read/write heads. In other types of DASDs, optical or other data storage media may be employed.
In a magnetic disk unit, the disks are formatted to define sectors and tracks upon the disk surfaces. Tracks are usually circular regions coaxial with the disk axis where data may be written, and sectors are parts of the tracks capable of storing a predetermined quantity of data written to the disk. Axially aligned tracks on the disks of a DASD are referred to as cylinders. The sectors of a DASD where blocks of data are stored have unique physical data block addresses (DBA). The disks of the DASD spin in unison around a common axis, and the read/white heads, usually one for each surface, are moved radially in unison across the disk surfaces. When data is read from or written to a physical DBA, the heads are moved into alignment with the cylinder containing the track in which the DBA is found, and the data transfer takes place as the sector or sectors of the DBA spin under the head.
Important considerations in the design and use of DASDs are capacity, speed of data transfer and reliability. For reasons including those discussed in Patterson et al., A Case for Redundant Arrays of Inexpensive Disks (RAID)xe2x80x3, ACM SIGMOD Conference, Chicago, Ill., Jun. 1-3, 1988, increasing performance levels attained by central processing units (CPUs) and system memory in data processing systems result in the need for larger and faster data storage systems. To achieve these goals of capacity and speed, arrays containing a number of DASDs have been used for data storage.
In a redundant array of independent disk drives (RAID), various modes of operation have been employed to effect reliability. In a RAID type 5 subsystem, a parity based correction is used. The array may include a number of N+1 DASDs. Blocks of data called stripes typically ranging from several sectors to a full DASD track are written on N of the DASDs. A parity block is formed by calculating the exclusive-or (XOR) of the N data blocks and storing this parity block on the remaining one of the N+1 DASDs in the array. The parity fields of the array are spread across all N+1 DASDs in the array. For each set of N blocks of data that are protected by a parity block, the location of the data and parity blocks is varied in a manner that distributes the parity blocks evenly across all DASDs in the RAID type 5 architecture.
A performance problem with the RAID type 5 subsystem known as the write penalty is due to the necessity to perform a minimum of four DASD operations for each host write operation to the array. Each data update requires reading the old data, writing the new data, reading the old parity data, XORing together the old data, new data and the old parity data to generate new parity data and writing the new parity data. System speed is impaired by the read, modify and write sequence.
In another array arrangement, backup or mirror devices have been used to provide data protection. With a RAID type 1 subsystem, a duplicate set of mirror storage units are used for storing a duplicate copy of all data on each pair of storage units. In the conventional mirroring RAID type 1 subsystem, there are at least two write operation per host write operation. Each data update requires writing the data to a data DASD and writing a copy of a mirrored DASD. While mirrored RAID type 1 subsystem provides high reliability, a disadvantage is the expense of the DASD devices required for duplicate data storage.
It is a principal object of the present invention to provide an apparatus and efficient method for processing data using a redundant array of independent disk drives (RAID) that overcomes many of the disadvantages of prior art arrangements. Another important object of the invention is to provide such apparatus and method that is efficient in the use of DASD resources minimizing read/write overhead activity to the DASDs in the array while providing data protection and cost benefit.
In brief, the objects and advantages of the present invention are achieved by apparatus and a method for processing data in data processing system including a redundant array of independent disk drives (RAID) operatively controlled by an array controller. A plurality of data drives and a predefined mirror drive are included in the RAID. The predefined mirror drive has a set capacity substantially greater than a capacity of each of the plurality of data drives. Data is written to and read from the multiple data drives in the RAID. A mirror copy of the data written on the multiple data drive is written on the predefined mirror drive in the RAID. A write cache is used with the array controller for temporary storage of host data to equalize system performance.